As is known, such a rigid material with closed cells may be obtained from a mixture containing polyvinyl chloride, a polyisocyanate, a suitable foaming agent, a vinylidene monomer and an ethylene anhydride, by molding under pressure and at a high temperature, followed by treating the molded product with hot water or with an atmosphere which is rich in water vapor which reacts with the polyisocyanate, the heat causing the molded product to expand until it attains the final dimensions required, and the product becoming hardened by completion of the water/polyisocyanate reaction.
This known reaction occurs according to the following formulas: ##STR1## Reaction of addition according to the reaction equation: ##STR2##
The mixture of the constituents is performed in an appropriate mixer, after which the mixture is introduced in a mold under pressure and at a high temperature of the order of 180.degree. C. During this molding stage there is no swelling (or blowing).
The molded embryo is then withdrawn from the mold under pressure and transferred into an expansion mold where it is submitted to the action of hot water or water vapor.
The blowing takes then place in the presence of hot water (see reactions above) which catalyzes the reaction to produce CO.sub.2 which causes expansion within the embryo.
Prior to the manufacture of rigid cellular materials of this type, flexible cellular PVC was prepared in accordance with the Carpentier U.S. Pat. No. 2,590,156 by carrying out a plural step operation which, briefly summarized, involved first forming a preform, termed a "miniature model" and heating the mixture under pressure to initiate gelatinization. Following this, the mass was expanded in an oven in a series of steps, and was finally cooled. The final product produced was soft.
The mold used in the Carpentier process of U.S. Pat. No. 2,590,156 is a so-called "elastic" mold which allows for expansion of the mass enclosed in the mold, the internal volume of the mold changing as the plastic material therewithin expands.
As the molding system of Carpentier 2,590,156 was suitable only for the manufacture of soft cellular PVC, it could not be used in conjunction with the rigid PVC of Carpentier U.S. Pat. No. 2,576,749. In French Pat. No. 946,719 the expansion is essentially carried out in two steps, namely a first step of expansion in the dry atmosphere to carry out the swelling of the mass, followed by a second step of expansion in an atmosphere saturated with water vapor in order to allow a further reaction to take place to increase the rigidity of the cellular body as noted above. The mold used is discussed only briefly, but is similar to that disclosed in Carpentier 2,590,156.
A second step in the evolution of the manufacture of rigid cellular PVC is disclosed in the aforementioned Carpentier U.S. Pat. No. 2,576,749, the French priority application of which was filed on May 9, 1947, the same date as the aforementioned French Pat. No. 946,719, and the French equivalent of which bears the number 946,720. In this patent, the expansion and rigidification are essentially carried out in only one step, which means that the mass to be molded is simultaneously exposed to a heat treatment and to the action of water vapor. Again the mold used is only very briefly described, but is of the same type as is disclosed in the earlier Carpentier patent 2,590,156.
A third step in the evolution of the procedure for the manufacture of rigid cellular PVC is disclosed in U.S. Pat. No. 3,290,262, which patent broadly mentions the concept of expansion in water. This concept is set forth in more detail in U.S. Pat. No. 3,283,043 which discloses a process and the apparatus for expansion in water. However, according to this patent the system permits only free expansion and not controlled expansion in the water. The apparatus shown for manufacturing rigid PVC includes a vat which may contain an expansion bath, suitable means for heating the bath, and means for manufacturing the cellular material immersed during the expansion stage, such maintenance means comprising either a system of grids which receives the material to be expanded and maintains it in immersed position during the expansion treatment, or vertical partitions which maintain the cellular plates to be expanded in the vertical direction during such expansion.
Accordingly, among the prior art processes for the production of rigid cellular PVC with sealed cells, it has been proposed to heat the mixture under suitable pressure in a mold which is gas-tight, and then transfer the resultant embryonic blocks into an apparatus constituted by a plurality of possibly perforated superimposed plates, which are then immersed in vats of hot water for a specific period of time to allow the blocks to expand until they reach final dimensions which correspond substantially to the length and width of each of the plates and the distance therebetween.
However, free expansion in hot water is not satisfactory and the very limited control achieved by the superimposed plates, whether perforated or not, is simply inadequate. When the embryonic blocks are permitted to expand freely in hot water, the final dimensions of the finished blocks are not controlled and the resultant rigid cellular PVC blocks are therefore irregular and erratic. This lack of regularity exists not only within a single block, but in addition the blocks differ from one another. The lack of homogeneity of the blocks expanded according to the free expansion process requires a shaping subsequent to the expansion operation, in order to put all the blocks back to the regular required dimensions; however, even the use of such an additional step does not remedy the irregularities in quality, and more particularly in rigidity, of the blocks.
According to the prior art, attempts have also been made to control the expansion of the processed embryonic blocks by enclosing them in frames closed by a cover and with a perforated metal base, the dimension of these being such that they allow the blocks to expand only in one dimension i.e. in thickness.
It has been demonstrated that the quality and finish of the rigid cellular materials may be greatly improved by carrying out the controlled expansion of the embryonic blocks in several stages, i.e. a pre-heating stage which consists of immersing the embryonic blocks in hot water at 80.degree.-85.degree. C. to obtain pre-expansion and homogenization of the temperature inside each of the blocks, then a first controlled expansion stage during which the pre-heated blocks are immersed in hot water at around 90.degree.-95.degree. C., and a second controlled expansion stage during which they are immersed in hot water at 95.degree.-100.degree. C. to undergo expansion to their final dimensions, these preheating and controlled expansion stages being limited in the three dimensions of the block by the walls of a closed frame in which the blocks are enclosed.
However, existing apparatus do not allow this improved process to be carried out except under very poor conditions, since they require a large number of manipulations and consequently the intervention of a considerable work force. Furthermore, this large number of manipulations has to be carried out in an atmosphere saturated with water vapor and therefore very unhealthy, so that the work force employed would be subjected to extremely unpleasant working conditions: in fact, the use of existing apparatus for carrying out the improved process involves the insertion of the embryonic blocks in the above-mentioned appliances, formed by a plurality of superimposed plates, the immersion of these appliances in vats of hot water for specific periods of time, after which the appliances are withdrawn from the vats, the pre-expanded blocks are taken out of the plates so that they can be subjected to a dimension check in order to remove blocks which are insufficiently foamed or are defective, then they are placed in molds with dimensions which are very close to those required for the final product; after such molds have been closed, they are, in turn, immersed in a vat of hot water for a specific period of time to allow the final expansion of the blocks to take place, controlled by the dimensions of the sealed molds, which are then immersed in vats of cold water to effect the dimensional stabilization of the sheets which have been expanded to their final dimensions, and then taken out of the vats of cold water and opened to allow the finished blocks to be recovered.
The existing apparatus does not allow the block expansion operation to proceed until the exact dimensions sought have been obtained, in a plate appliance and/or a single frame appliance, since if it is attempted to carry out the expansion of the embryonic block obtained during the preliminary molding operation on a plate or in a frame which has the final dimensions sought, the finished blocks have irregular dimensions and qualities: in fact, the dimensions of the embryonic blocks are substantially less than the dimensions of the plate which supports them or of the closed frame in which they are enclosed, which correspond with the final dimensions required for the expanded block, and with the existing apparatus it is not possible to prevent a possible displacement of the embryonic block on the plate or in the frame during the expansion operation in water, which displacement would have an effect on the dimensional expansion of the block and would therefore give rise to finished blocks with irregular dimensions and qualities.